The OPAL Discussion Forum

[Christof Metzger-Kraus <christof.j.kraus AT gmail.com>]

Hi Ryan,

when the particles are emitted from a cathode (EMITTED = True), then you should set the attribute EKIN (in eV) of the DISTRIBUTION command. When the particles are injected then you should set PC of the BEAM command. The attributes of BEAM expect equivalents of GeV.

The attribute OFFSETPZ of the DISTRIBUTION command should in most cases be 0. With it one can model a beam that has a momentum that differs from the nominal momentum.

I agree that the situation of units should be improved.

Best regards

christof

On Thu, Mar 15, 2018 at 1:09 AM, Ryan Roussel <roussel AT g.ucla.edu> wrote:

Hello all,

I am trying to understand how OPAL handles particle energy and momentum. I've seen two different methods for specifying a beam distribution with a non-zero initial energy. One can specify the central momentum/energy/gamma in the BEAM command or I have seen examples where a non-zero offset can be specified in the DISTRIBUTION command. 

I ran the same input file using both of these methods and I have been unable to interpret the results. In the case where I specify the energy only in the BEAM command I see the energy correctly specified in the BEAM portion of the OPAL-T output. But when I look at the final bunch dump BUNCH region, the kinetic energy is not correct. Furthermore it takes orders of magnitude more time to travel 0.1 m in a simple drift than it should.

Conversely, when the energy is specified in both the DISTRIBUTION command and the BEAM command the kinetic energy in the BUNCH dumps is correct and the amount of time to travel 0.1 m seems correct but the "spos" and reference trajectory are wrong.

Part of my confusion could come from units issues, where the energy and momentum specifications are in eV,Mev,or GeV in both the program and documentation. As a new user, this is difficult to work with and would be a request of mine to improve the code unless there is a major reason for this.

Regardless, this is something that I could use help with. I have attached the input file and the outputs from each run that I discussed before. The beamline is a simple drift with a screen.

Thanks for you time,

Ryan Roussel